The common processes in fiber-dyeing can be roughly summarized as follows: (1) applying dyes dissolved in water to fibers evenly; (2) applying heat to react fibers and dyes; (3) removing the unreacted dyes by washing the fibers with a large amount of water; and (4) drying the fibers. These processes are repeated as needed.
In the fiber-dyeing processes, one of the important elements is water. A large amount of water is used and a large amount of wastewater is discharged. To prevent environmental pollution, it is desirable to clean the wastewater. However, since the dyes are intrinsically not highly water-soluble and have a high absorption coefficient, most cleaning methods are expensive and energy-consuming, such as adsorption with activated carbon or distillation.
Many dye plants treat their wastewater using a combination of the coagulation-sedimentation method and the activated sludge method. However, these methods can only remove dyes that are relatively easy to remove, and other dyes remain discharged in the wastewater. Also, in these methods, the excess sludge, a by-product of water treatment, then has to be safely disposed.